1. Field of the Invention
The present invention relates to an assembled battery having multiple cells connected in series, a power-supply system using the same, and a method of producing the assembled battery.
2. Description of the Related Art
Recently, assembled batteries having multiple cells connected have been used in large-scale power-supply systems used, for example, in electric vehicle, hybrid electric vehicle, and home, to obtain desirable power. Non-aqueous secondary cells such as lithium-ion secondary cell and lithium polymer secondary cell, which are light and higher in capacity and power, are attracting attention as a battery for the large-scale power-supply systems.
The battery for use in such an application as large-scale power-supply system is preferably a battery that can be operated at a wider depth of charge and discharge, and large fluctuation of its power is undesirable for stabilized supply of power.
The power of battery is calculated by (voltage of battery)×(current), or (voltage)×(voltage)÷(internal resistance of battery), and thus, depends largely on the voltage or internal resistance of the battery. Non-aqueous secondary cells including lithium-ion secondary cell generally have a higher battery voltage and a lower internal resistance when the depth of charge is greater or the depth of discharge is smaller. On the contrary, when the depth of charge is smaller or the depth of discharge is greater, the battery voltage becomes lower and the battery internal resistance greater. Thus, by reducing the difference in battery voltage or internal resistance caused by depth of charge and discharge, it is possible to reduce the power fluctuation even when the cell is operated at a wide range of depth of charge and discharge.
When multiple non-aqueous secondary cells are connected and used as an assembled battery, the cell is operated at a middle range of depth of charge and discharge, in order, during discharge, to ensure discharge reserve for preventing overdischarge of a cell having a smaller capacity due to fluctuation in capacity of each cell and in order, during charge, to prevent deterioration of a cell due to phase change of a positive-electrode active material by high voltage. For example, in a common lithium-ion secondary cell employing a cobalt-based oxide as a positive-electrode active material and graphite as a negative-electrode active material, use in a depth of charge and discharge in the regions close to the upper and lower limits is avoided, and instead, the used region of the depth of charge and discharge is normally in the range of 25 to 75% excluding about 25% range respectively in the upper and lower limits of depth of charge and discharge.
Methods of integrating battery temperature or current, for example, are known as the method of determining the depth of charge and discharge of such a secondary cell. However, these methods often cause error during long-term use, and for that reason, it is also determined from the battery voltage or internal resistance. The internal resistance is determined from the change in the current ΔI flowing in battery and the change in the voltage ΔV of battery, according to the Formula: [Internal resistance=ΔV/ΔI].
On the contrary to the case for reduction of power fluctuation, when the depth of charge and discharge is to be determined from the battery voltage or internal resistance, the battery voltage or internal resistance preferably varies according to the depth of charge and discharge in the region of the depth of charge and discharge used. However, it is difficult to determine the depth of charge and discharge from battery voltage or internal resistance, when a non-aqueous secondary cell, which shows a smaller change in voltage or internal resistance, is used. In particular, non-aqueous secondary cells employing a 5V-class spinel lithium manganese oxide LiNi0.5Mn1.5O4 or an iron phosphate compound as the positive-electrode active material have a very flat charge/discharge curve, flatter than that employing conventional positive-electrode active materials, and thus, are highly expected as a battery with smaller power fluctuation. Thus, an assembled battery comprising non-aqueous secondary cells containing such a positive-electrode active material shows smaller power fluctuation in a wider range of depth of charge and discharge and is superior in stability, but it becomes more difficult to determine the depth of charge and discharge by a method of detecting the voltage or internal resistance of battery.
Generally when a non-aqueous secondary cell is used to the region smaller or greater in the depth of charge and discharge, detection of the voltage or internal resistance of battery may become easier, but practically, the battery is not always used to both limits of the depth of charge and discharge. In addition, in an assembled battery including multiple cells, part of the cells may be overcharged or overdischarged, because of fluctuation in capacity of the cells. For that reason, use of the battery to the regions smaller and greater in depth of charge and discharge is undesirable. A detection circuit may be installed for each cell in the assembled battery for accurate detection of the voltage or internal resistance of the cell, but such a method makes the apparatus more complicated and demands a high-accuracy detection element, causing a problem of increase in production cost.
Accordingly, there are some methods proposed for such assembled batteries in which multiple non-aqueous secondary cells are connected. For example, Patent Document 1, Japanese Unexamined Patent Publication No. 9-180768, proposes a method of using an assembled battery containing mainly non-aqueous secondary cells and an aqueous-solution secondary cell, such as nickel metal-hydride cell, having a capacity smaller than that of the non-aqueous secondary cells connected thereto. In the assembled battery, charging of the non-aqueous secondary cell is terminated before it is overcharged, because the late charging state is detected by the aqueous-solution secondary cell having a smaller capacity. In addition, the aqueous-solution secondary cell shows a characteristic voltage phenomenon different from that of a non-aqueous secondary cell in the late charging state, allowing determination of the depth of charge of the entire assembled battery only by determining the voltage of the aqueous-solution secondary cell.
However, the assembled battery disclosed in Patent Document 1 has a problem that the relationship between the setup depth of charge and discharge and the voltage varies gradually during use.
Specifically, the method of voltage detection disclosed in Patent Document 1, which is aimed at measuring the large voltage change occurring at the end of charging in an aqueous-solution secondary cell, allows detection of voltage in the late charging state. However, in aqueous-solution secondary cells, the charge current flowing in the overcharge range is not used for charging, but used for electrolysis of water in the electrolytic solution, and non-aqueous secondary cells, which are not charged completely, remain to be charged, even when the aqueous-solution secondary cell is overcharged. In addition, in aqueous-solution secondary cells, part of the charge current is not used for charging but used for electrolysis of water also in the normal use region, for example, when large charge current flows or the environment temperature is high. Accordingly, even when the assembled battery is produced in such a setting that the uncharged state of an aqueous-solution secondary cell and the uncharged state of a non-aqueous secondary cell are harmonized, the relationship between the depth of charge and discharge of the aqueous-solution secondary cell and that of the non-aqueous secondary cell varies soon relatively easily. In particular, in the assembled battery of Patent Document 1, in which the capacity of the aqueous-solution secondary cell is made smaller than that of the non-aqueous secondary cell, the aqueous-solution secondary cell is always charged to the overcharge region, which may result in deterioration in capacity, and the deterioration in capacity also leads to deviation of the depth of charge and discharge. It also causes a problem of impairing the function to control charge and discharge of the entire assembled battery while predicting the charge state of the non-aqueous secondary cell by using the voltage drop occurring when the aqueous-solution secondary cell is charged completely.
It is occasionally difficult to detect the depth of charge and discharge from battery voltage, depending on the condition of battery operation. For example, when a battery is always in operation of supplying current, the voltage varies according to the intensity of the current, resulting in inconsistency between the battery voltage and the depth of charge and discharge. For example, Patent Document 2, Japanese Unexamined Patent Publication No. 2000-21455, proposes a method of measuring the current and voltage of a battery, determining the internal resistance of the battery from the observed values, and thereby, correlating the internal resistance with the depth of charge and discharge of the battery.
The method of correlating the internal resistance with the depth of charge and discharge of a battery disclosed in Patent Document 2 allows easy detection of the depth of charge and discharge if the battery used is a battery having a large internal resistance. However, it is difficult to determine the depth of charge and discharge from the internal resistance, when non-aqueous secondary cell having very small change in battery internal resistance, which is favorable as a power source in power-supply system, is used. In particular, in the case of a non-aqueous secondary cell employing a 5V-class spinel lithium manganese oxide LiNi0.5Mn1.5O4 or an iron phosphate compound as the positive-electrode active material, the method of detecting battery internal resistance causes a problem of difficulty in determining the depth of charge and discharge, because such a cell shows an internal resistance change by depth of charge and discharge smaller than that of the non-aqueous secondary cell employing a conventional positive-electrode active material such as nickel oxide or cobalt oxide.